Being continuously exposed to variable environmental conditions, plants produce phytohormones to react quickly and specifically to these changes. The phytohormone ethylene is produced in response to multiple stresses. While the role of ethylene in defense responses to pathogens is widely recognized, recent studies in arabidopsis and crop species highlight an emerging key role for ethylene in the regulation of organ growth and yield under abiotic stress. Molecular connections between ethylene and growth-regulatory pathways have been uncovered, and altering the expression of ethylene response factors (ERFs) provides a new strategy for targeted ethylene-response engineering. Crops with optimized ethylene responses show improved growth in the field, opening new windows for future crop improvement. This review focuses on how ethylene regulates shoot growth, with an emphasis on leaves.

Drought stress is a major problem for agriculture worldwide, causing significant yield losses. Plants have developed highly flexible mechanisms to deal with drought, including organand developmental stage-specific responses. In young leaves, growth is repressed as an active mechanism to save water and energy, increasing the chances of survival but decreasing yield. Despite its importance, the molecular basis for this growth inhibition is largely unknown. Here, we present a novel approach to explore early molecular mechanisms controlling Arabidopsis leaf growth inhibition following mild drought. We found that growth and transcriptome responses to drought are highly dynamic. Growth was only repressed by drought during the day, and our evidence suggests that this may be due to gating by the circadian clock. Similarly, time of day strongly affected the extent, specificity, and in certain cases even direction of drought-induced changes in gene expression. These findings underscore the importance of taking into account diurnal patterns to understand stress responses, as only a small core of drought-responsive genes are affected by drought at all times of the day. Finally, we leveraged our high-resolution data to demonstrate that phenotypic and transcriptome responses can be matched to identify putative novel regulators of growth under mild drought.

Leaf growth is a tightly regulated and complex process, which responds in a dynamic manner to changing environmental conditions, but the mechanisms that reduce growth under adverse conditions are rather poorly understood. We previously identified a growth inhibitory pathway regulating leaf growth upon exposure to a low concentration of mannitol and characterized the ETHYLENE RESPONSE FACTOR (ERF)/APETALA2 transcription factor ERF6 as a central activator of both leaf growth inhibition and induction of stress tolerance genes. Here, we describe the role of the transcriptional repressor ERF11 in relation to the ERF6-mediated stress response in Arabidopsis (Arabidopsis thaliana). Using inducible overexpression lines, we show that ERF6 induces the expression of ERF11. ERF11 in turn molecularly counteracts the action of ERF6 and represses at least some of the ERF6-induced genes by directly competing for the target gene promoters. As a phenotypical consequence of the ERF6-ERF11 antagonism, the extreme dwarfism caused by ERF6 overexpression is suppressed by overexpression of ERF11.Together, our data demonstrate that dynamic mechanisms exist to fine-tune the stress response and that ERF11 counteracts ERF6 to maintain a balance between plant growth and stress defense.Plants are constantly challenged to survive and maintain growth in changing environments. In natural environments, as well as in laboratories, growth conditions are rarely optimal, generating a weak but continuous stress. In such suboptimal conditions, the equilibrium between sustained plant growth and activation of stress defense mechanisms is defied and needs to be continuously rebalanced and fine-tuned (Claeys and Inzé, 2013).To unravel these growth-and defense-related mechanisms in Arabidopsis (Arabidopsis thaliana), researchers commonly use in vitro setups in which different growth inhibitory compounds are added to the growth medium (Verslues et al., 2006;Lawlor, 2013;Claeys et al., 2014). Mannitol, for example, is a frequently applied compound to induce mild stress because it results in both inhibition of leaf growth and activation of stress-responsive genes (Kreps et al., 2002;Skirycz et al., 2010Skirycz et al., , 2011Dubois et al., 2013;Claeys et al., 2014;Trontin et al., 2014). Two putative receptorlike kinases, ENHANCED GROWTH ON MANNITOL1 (EGM1) and EGM2, are presumably involved in the detection of mannitol and further downstream activation of the growth and tolerance responses (Trontin et al., 2014). Previously, we have shown that mannitolinduced responses are specific to the different stages of Arabidopsis leaf development (Skirycz et al., 2010). In very young Arabidopsis leaves, in which cells are not yet expanding but still actively dividing, exposure to mannitol triggers the accumulation of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and the transcriptional induction of ethylene-related genes. Interestingly, these responses are extremely fast, with several ETHYLENE RESPONSE FACTORs (ERFs; ERF1, ERF2, ERF5, ERF6, and ERF11...